The present invention relates to radio communications systems, and more particularly to duplex schemes in time-division multiple-access (TDMA) systems.
Most time-division multiple-access wireless communications systems employ either a time-division duplex (TDD) scheme or a frequency-division duplex (FDD) scheme to separate uplink and downlink transmissions. Since both duplex schemes provide certain advantages and disadvantages, both schemes are routinely utilized in wireless communications applications.
For example, in the Personal Wireless Telecommunication (PWT) standard, time-division multiple-access with time-division duplex is used for frequency planning as well as signal packet and time slot assignment. Such a time-division multiple-access/time-division duplex scheme is well suited for many business wireless communication applications (e.g., small-campus systems with micro or pico cells).
On the other hand, time-division multiple-access with either time-division duplex or frequency-division duplex can be preferable for licensed Personal Communication Service (PCS) frequency bands, depending upon customer demands and marketplace requirements. In other words, since the structure of a Personal Communications Service system is primarily determined by a service provider having acquired a portion of the frequency spectrum, the technology and frequency usage implemented in such a system is ultimately driven by customer demand as well as legal and practical constraints. While a first customer may request a time-division multiple access/time-division duplex system for a particular business wireless application, a second customer may thereafter demand a time-division multiple access/frequency-division duplex system for a wireless local loop application.
Thus, product and service providers are often required to convert between duplex schemes. Converting between schemes, however, typically results in duplicated effort and therefore wastes significant time and resources. For example, since the conventional time-division duplex and frequency-division duplex schemes are fundamentally different, it generally is not feasible to use a common hardware platform for both types of system. As a result, two development teams are typically assigned, and two separate product lines are usually established, to provide for both time-division duplex and frequency-division duplex implementations.
Thus, there is a need for a flexible duplex scheme which will allow a communications system to be adapted to satisfy varying customer needs without requiring modification of basic system hardware architecture.
The present invention fulfills the above-described and other needs by providing a flexible division duplex mechanism in a time-division multiple-access communications system. More specifically, the disclosed system utilizes a mixed, or hybrid, division duplex mechanism such that uplink and downlink transmissions are separated in frequency while time slots associated with transmission and reception are also separated in time. The hybrid duplex scheme, referred to herein as frequency-time division duplex (FTDD), allows alternative division duplex mechanisms to be selectively implemented within a communications system without requiring modification of the basic system hardware architecture.
Advantageously, the disclosed system can utilize a single hardware platform for applications where either time-division duplex or frequency-division duplex is preferred. According to exemplary embodiments, the disclosed system is neither a pure time-division duplex system, in which the same frequency band is used for both uplink and downlink transmissions, nor a pure frequency-division duplex system in which both uplink and downlink transmissions occur simultaneously. Rather, the disclosed system utilizes separate frequency bands as well as separate time slots for uplink and downlink communications. Thus, a hardware platform initially designed for use in a time-division multiple access/time-division duplex system can be readily adapted for use in a time-division multiple access/frequency-division duplex system, and vice versa, without significant hardware modification. This feature of the present invention results in lower non-recurring engineering costs and shorter system development time.
Furthermore, since uplink and downlink communications are separated in both frequency and time, the disclosed system provides less cross-channel interference as compared to prior art systems. Also, since a single hardware path can be used for both uplink and downlink transmissions at both base stations and terminals, embodiments of the present invention retain the advantages of low cost and power consumption typically associated with conventional time-division duplex systems. The present invention also provides methods and apparatus which enable a frequency-time division system to operate without loss of spectral efficiency.
According to an exemplary embodiment, a wireless communications system includes a plurality of mobile stations and a base station. The base station is configured to transmit downlink communications signals to the mobile stations via a first carrier frequency and to receive uplink communications signals from the mobile stations via a second carrier frequency, the downlink and uplink communications signals being transmitted and received via successive time division multiple access frames, each frame including a plurality of time slots. For each active communications link (e.g., for each call) established between the base station and a mobile station, a first time slot within each frame is allocated for downlink communications and a second time slot within each frame is allocated for uplink communications, the first and second allocated time slots being separated in time by a fixed time offset. According to the embodiment, a first partition of the time slots within each frame is reserved for downlink communications from the base station to the mobile stations and a second partition of the time slots within each frame is reserved for uplink communications from the mobile stations to the base station.
The communications system can also include an additional base station co-located with the first base station and similarly configured to transmit downlink communications signals to the mobile stations via the first carrier frequency and to receive uplink communications signals from said mobile stations via the second carrier frequency, the downlink and uplink communications signals of each of said additional base stations being transmitted and received via successive time division multiple access frames, each frame including a plurality of time slots. For each active communications link established between the additional base station and a mobile station, a first time slot within each frame of the additional base station is allocated for downlink communications from the additional base station and a second time slot within each frame of the additional base station is allocated for uplink communications to the additional base station, the first and second allocated time slots being separated in time by a fixed time offset. A first partition of the time slots within each frame of the additional base station is reserved for uplink communications and a second partition of the time slots within each frame of the additional base station is reserved for downlink communications. To provide full time and spectral efficiency for the coverage area serviced by the two co-located base stations, the first and second partitions of the frames of each of the base stations are structured such that, for each time slot in each frame, only one of the base stations is permitted to transmit on the first carrier frequency and only one of the base stations is permitted to receive on the second carrier frequency.